AVS2013 Session SS-TuA: Metals and Alloys: Structure, Reactivity & Catalysis

Two forms of the reaction intermediate with H:C:O stoichiometry were studied on Pt(111) using density functional theory. Three cases were studied: the intermediate on a clean surface, the intermediate and one water molecule on the surface and the intermediate with a water bilayer. Both the H-C=O (formyl) and C-OH configurations were found to be stable on clean Pt(111) and when coadsorbed with a single water molecule. On the clean surface interconversion between the two forms goes through stable a CO_ads and H_ads intermediates so although both HCO and COH are stable on the clean surface they will not interconvert without coadsorbed water. In the presence of coadsorbed water molecule the activation barrier for the interconversion from HCO to COH was found to be much lower or 0.62 eV. The HCO configuration is stable in the presence of a water bilayer with slight preference for bridge over atop adsorption site. The COH configuration is unstable under the water bilayer and dissociates into CO_ads and H that structurally diffuses into the water bilayer. That suggest that, in the presence of water, the HCO form will be the only stable form of the reaction intermediate of H:C:O stoichiometry. Various reactions paths for both of the reaction intermediates with OH and H₂O eventually leading to CO or CO₂ formation are also investigated.

We used reflection absorption infrared spectroscopy (RAIRS) to experimentally study the kinetics of the NH formation and dissociation reactions on Ru(0001). These reactions are important in ammonia synthesis, for which ruthenium is one of the most effective catalysts. While theoretical values for the heats of adsorption of each species and the activation barriers for each step in the ammonia synthesis reaction have been extensively reported, there are few, if any, direct experimentally measured values for the activation barriers of the elementary steps in the reaction on Ru(0001). In our study, the evolution of the NH stretch peak at 3318 cm^-1 was monitored as a function of time during the course of the NH formation reaction between 320 and 370 K and NH dissociation between 370 and 400 K. An atomic N layer was prepared through the thermal decomposition of NH₃. During the NH formation reaction, the sample was continuously exposed to a constant background pressure of H₂ to compensate for the loss of surface H due to the associative desorption of H₂, which occurs in the same temperature range as NH formation. Both NH formation and dissociation were found to follow first-order kinetics, and the experimental rate constants show good agreement with the theoretical values. The experimentally determined activation energies of 69.6 and 104.9 kJ/mol for NH formation and dissociation, respectively, are comparable to the theoretical values of 103.3 and 133.5 kJ/mol.

Surface science has a long and illustrious history. The development of surface science and of the AVS have often intersected, and even coincided. In this talk I hope to discuss some of these intersections and coincidences.

The nucleation, growth and surface composition of Pt-Co and Pt-Re bimetallic clusters on TiO₂(110) have been investigated as model systems for understanding how surface chemistry can be controlled by bimetallic composition and interactions between the clusters and the oxide support. Scanning tunneling microscopy studies demonstrate that bimetallic clusters can be formed from sequential deposition of the metals when the metal with the lower mobility on titania (Co or Re) is deposited first to serve as nucleation sites for Pt. For the Co-Pt clusters, the surface composition is slightly Pt-rich compared to the bulk, despite the lower surface free energy for Co compared to Pt. While Co-Pt clusters have similar surface compositions regardless of the order of deposition, the inter-diffusion of metals in Re-Pt clusters is inhibited when Pt is deposited on top of Re; for the reverse order of deposition, XPS data indicate that a Re-Pt alloy is formed. Temperature programmed desorption experiments with CO show that CO adsorbs at both Co and Pt sites on the bimetallic clusters. Methanol decomposition chemistry has also been studied as a function of changing bimetallic cluster composition. Methanol decomposition produces CO and hydrogen as the main products on both pure Pt and pure Co clusters. On the bimetallic Co-Pt clusters, the selectivity for methane production increases, and the desorption of H₂ at higher temperatures suggests that the intermediates have greater thermal stability.

Fischer-Tropsch synthesis (FTS) has recently gained increased attention as it involves the formation of hydrocarbons (fuels) via the catalytic conversion of syngas (CO and H₂), which can be derived from renewable sources. FTS is often performed using cobalt-based catalysts, and although the exact mechanism of the reaction is not known, it has been shown that the reactivity is affected by the adsorption state of reactants, as well as nanoparticle shape and size. Here we have used low-temperature scanning tunneling microscopy to study carbon-carbon bond formation during FTS. By using aryl-halides to form phenyl radicals on the cobalt surfaces, we can examine the preferred site for carbon-carbon bond formation. Initially at 5 K, the intact molecules form loosely-ordered arrays on the cobalt surfaces, but at 80 K, we find that phenyl radicals are stabilized in a series of highly ordered geometries on the cobalt terraces. Upon heating, a carbon-carbon bond is formed that results in surface-bound biphenyl. These results provide insight into the active site for carbon coupling during FTS, and we propose a mechanism that may explain the well-known phenomenon of surface roughening that occurs on cobalt surfaces under real FTS conditions.